The Ontario Ministry of Education recently implemented the Steps to English Proficiency (STEP) language assessment framework to build educator capacity for addressing the needs of English language learners (ELLs) in K-12 schools. The STEP framework is a set of descriptors-based language proficiency scales that specify observable linguistic behaviours from which educators can make inferences about students' English language development. Teachers use these proficiency scales to assess, document, and track students' language proficiency development based on daily interactions with students in classrooms. The purpose of this article is to report on teachers' perceptions of and experiences with the STEP proficiency scales during a three-year pilot implementation and validation study of the initiative. Based on analysis of these findings, we articulate implications for building teachers' assessment capacity using observational language assessment scales for K-12 ELLs.